1. Field of the Invention
The invention concerns a gating device to delimit an x-ray beam, of the type having two absorber elements disposed opposite one another that are adjustable with an adjustment device as to their distance from one another, via which a slit for passage of the x-ray beam can be delimited, wherein the absorber elements are shaped such that the slit exhibits a slit width varying in the slit longitudinal direction, the slit width increasing toward both slit ends starting from a middle position. The invention also concerns a computed tomography apparatus with an x-ray radiator rotatable around a system axis, with an x-ray detector and with a radiator-side gating device.
2. Description of the Prior Art
In an examination of an examination subject or a patient in an x-ray diagnostic apparatus, the examination subject is moved into an x-ray beam emitted by an x-ray source, and the radiation attenuation resulting from this is detected by an x-ray detector. The examination subject is thus located in the beam path between the x-ray receiver and the x-ray detector. The typical x-ray tubes used as x-ray radiators radiate x-ray radiation in a significantly larger solid angle than is necessary for examination at the patient. In order to prevent an unnecessary radiation exposure at the patient, the necessity thus exists to gate out unnecessary x-rays. For this purpose, in conventional x-ray apparatuses it is known to dispose a radiator-side gating device immediately after the x-ray radiator in the beam path, which gating device is also designated as a primary beam diaphragm. For example, such a primary beam diaphragm, with diaphragm plates which can be moved opposite to one another as absorber elements, is known from EP 0 187 245 A1.
In computed tomography apparatuses with multi-row x-ray detectors, a detector-side beam diaphragm (or a beam diaphragm near to the detector) that is mounted in the beam path between the patient and the x-ray detector is also frequently used in addition to a radiator-side gating device that is arranged in the beam path between the x-ray radiator and the patient. It is thereby possible to shade one or more detector rows of the multiple detector rows that are present and to use the remaining detector rows as active detector rows. Since, in a computed tomography apparatus (in particular in such a computed apparatus of the third generation), the x-ray detector rotates around the patient together with the x-ray radiator mounted on a gantry (rotating frame), the control and/or regulation device is normally curved in the azimuthal direction. In adaptation to this geometry, in particular in order to realize a constant separation, a detector-side diaphragm disclosed in DE 42 26 861 C2 for a computed apparatus is fashioned with arc-shaped diaphragm plates.
It is intended that the radiator-side diaphragm only passes such rays that can actually be detected by the x-ray detector (and in particular by its active detector rows). Other x-rays would only unnecessarily penetrate the patient and unnecessarily increase the radiation exposure. Since the multi-row x-ray detector arrays in computed tomography apparatuses are normally equipped with orthogonal rows and columns of detector elements, with regard to the primary beam diaphragm the object is to gate an exactly rectangular ray beam. In other words the resulting slice profile should assume the desired shape and half-width value. Given conventional flat or planar diaphragm plates or absorber elements, this is not perfectly possible due to different separations of the x-rays of the ray beam, respectively measured from the focus of the x-ray radiator to the point of impact on the diaphragm plate. To prevent disadvantageous edge effects in the gating, U.S. Pat. No. 6,396,902 an x-ray collimator is specified in which a number of slits of different but respectively constant width are introduced in a carrier or base body, whereby the carrier body is curved such that the gating slits are also curved. By the curvature of the slits, it should be ensured that a ray beam (dose profile) exactly rectangular in cross section is gated on the x-ray detector.
For different examination methods, in order to be able to operate with different numbers of active detector rows or with an x-ray beam gated at different widths in the direction of the patient axis, given the x-ray collimator known from U.S. Pat. No. 6,396,902 the entire support body produced from x-ray-absorbing material must be moved. According to this disclosure, this occurs by rotation of the bearing body, which is why the support body is also curved around a second axis (shell-shape). In order to also be able to bring another diaphragm slit into the matching position, the rotation axis would have to be located at the height of the focus of the x-ray radiator. This at best possible with very large mechanical effort.
Alternatively, the rotated support body would have to be readjusted into the correct position via a shifting movement, which is likewise very elaborate.
Moreover, the production of a support body curved around two axes is likewise connected with large expenditure, because this must also still be produced from x-ray-absorbing material, meaning from a material with a high atomic number. A further disadvantage of the x-ray collimator known from U.S. Pat. No. 6,396,902 is its large structural volume.